I know Romans had rotations to relieve the front lines but I'm trying to imagine how armor-clad medieval foot soldiers could last more than 10 minutes of close combat without being overcome with exhaustion. Adrenaline is one thing but what did they do in the open field? Did the soldier who monitored his stamina and escape combat to rest survive over those who didn't?
I know Romans had rotations to relieve the front lines
If you're thinking about rotating out the ranks as portrayed in the first episode of Rome...I'm afraid that's Not A Thing. As for how combat worked, while physical conditioning does help, the real problem here is the one you're thinking: nobody, no matter how well-conditioned, can last the entirety of a battle if they were constantly fighting, and yet we know that battles frequently took hours to resolve. Our best model so far is the 'pulse theory', outlined in these previous posts:
As always, if anyone else would like to write an answer of their own, please don't let this post stop you!